9v Battery Status Indicator Circuit

Introduction: 9v Battery Status Indicator Circuit

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This is a 9v battery status indicator circuit.One of my friend asked me to build this as he wants a 9v transistor radio battery status indicator. Thats why i build this circuit it works great and another thing about this circuit by changing the 'ZENER diode" you can rebuild this circuit for other low power battery. I used Green and Red led as green Led light's up when the voltage is upper than 6.9v and Red led lights up when the voltage drops to 6.9v . Its simple and reliable all the parts are commonly available and low cost also.

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25 Discussions

Hi @victordas, I haven't built the circuit but viewing the comments below and analysing the circuit myself by pen and paper I don't see how the red light will ever be switched off. Can you please explain it to me?What I understand so far is that when the battery voltage exceeds the zener voltage + base emitter voltage of the BJT (6.2 + 0.7 = 6.9V) the zener will reverse breakdown and there will exist small current that is able to enter the base of the first transsitor, which allows current to flow from the collector to the emitter. This current must first pass through the green light, which turns it on. I also understand that when the battery voltage is less than 6.9V the first transistor will not allow current to flow through to its emitter, and due to the 47k resistor there will be only a very small current through the green LED into the base of emitter 2 - not enough for it to visibly be turned on - which in turn allows current to flow through from collector to emitter of transistor 2 and causes red LED to glow.

However, can there not also be small current that has passed through the green light which enters the base of emitter 2 (as the base voltage is 0.7V and there still exists a voltage differential across the 47k resistor) which will also allow current to flow from collector to emitter of transistor 2 and cause the red LED to glow.

First time, I build this circuit, I also faced a problem, where two LEDs lit up when voltage is above than set value which determined by zener diode. So, I make some modifications on its base resistor at transistors, and it worked out. Hope it will help you guys! Done with Proteus 8

- I would like to place 3 different LEDs on my circuit to indicate the charge level for a 4 AA rechargeable battery pack. They all together and completely charge would give me 4.8 volts, which Zener diode do you suggest to me?

- Could I use 1 zener diode and different resistors values to light up the different leds?